An example of a magnetic media platter with embedded servo fields is illustrated in FIG. 1. These servo fields may also be referred to as servo bursts or servo marks. Each concentric circle schematically represents one track which is subdivided into multiple sectors and each radial line extending from the center outward, represents a servo field. A servo field contains data which is pre-written on the disk, during manufacturing of the disk, concerning the location of the servo field and the track on the disk for positioning of the read/write head relative to a particular track. The servo fields are then used by the controller for positioning of the read/write head during read and write operations. It is imperative therefore, that the data within the servo fields not be written over. In order not to write over the data within a servo field, the controller must know where the servo fields are and be able to suspend write operations over that area.
The usable regions for storage on the disk are located on the tracks, between the servo fields. Data to be stored on the disk is written in these regions between the servo fields, which will be referred to as a data region. The recording of data within these regions is measured in bits per inch (BPI). Assuming a constant bit density to record a bit, the number of bits which can be recorded between servo fields is much less for the inner circle, track 0, than the number of bits which can be recorded in the outermost circle, track N. Though possible, it is impractical to provide a unique recording and reading speed for each track. In order to take advantage of the higher storage potential of the outer tracks without exceeding the allowable density on the inner tracks, the disk is divided into multiple zones and a method of zone bit recording is used. The tracks are grouped into zones such that all the tracks in one zone are adjacent to each other. The data rate at which data is written to or read from the disk is constant for all tracks within a zone and different for each zone. Because the disk is rotated at the same speed for all the zones, in order to take advantage of the greater recording area of the outer zones, the recording speed is increased for the outer zones on the disk.
The data to be stored on a disk is broken up into manageable portions called sectors. Multiple sectors are generally stored on each track. An example of a typical format of a sector is illustrated in FIG. 2. Each sector includes an ID field, a GAP field, a DATA field and a PAD field. The ID field is the unique identification tag for each sector in a track of a disk drive media and differentiates one sector from another. Within the ID field are four subfields, the ID PLO subfield, the ID AM subfield, the ID subfield and the CRC subfield. The ID PLO subfield is a multiple byte field of a known pattern which is used by the phase lock loop of the encoder/decoder (ENDEC) to synchronize to the incoming data stream. The ID AM subfield is a known pattern which is used by the drive controller to synchronize or align to the beginning of the ID subfield. This synchronization is necessary to align the disk controller to a data byte boundary, in order to differentiate each segment of data to a particular field. The ID subfield follows the ID AM subfield and contains the actual identification for the sector, including multiple bytes used to specify the address of the sector on the disk. The number of bytes within the ID subfield is at the discretion of the manufacturer of the disk drive and is controlled by the format of the particular disk drive used. The number of bytes within the ID subfield is the same for every sector and can comprise a cylinder high byte, a cylinder low byte, a sector number byte, a head number byte and a byte or two for defect management. The ID subfield is then followed by a fixed number of error detection bytes in the CRC subfield which are used to detect any errors in the ID field. Some formats, referred to as ID.sub.-- Less formats, do not use an ID field, but rather use a header subfield within the DATA field.
The GAP field is a fixed number of bytes which are used to separate the ID field from the DATA field. The DATA field includes a DATA subfield and an error correction code (ECC) subfield. The DATA subfield is the portion of the sector where the actual data, which is communicated between the host computer and the disk drive, is stored. The ECC subfield is a fixed number of bytes tagged on to the end of the DATA subfield which are used to detect and correct soft or hard errors on the media within the capability of the code. This is necessary in order to avoid the transfer of erroneous data to and from the host computer.
An ID.sub.-- Less sector format of the prior art is illustrated in FIG. 2b. In ID.sub.-- Less formats, the ID field is replaced by a header subfield within the DATA field, thus combining the identification data and the DATA field into one field and reducing the number of bits necessary for each sector. The header subfield can be brought under the protection of the same ECC field as the DATA field and therefore afforded the same protection as the DATA field. The CRC field associated with the header subfield can be decreased or eliminated, further reducing the overhead of the system and eliminating the hardware which generates the CRC field. In ID.sub.-- Less formats all of the information in the sector header may be predetermined by reading a small number of other sector headers on the track, usually one. The sync field or PLO field in an ID.sub.-- Less format can be reduced in size or the reliability of the sync circuitry improved since the sync field will always occur immediately after a sector pulse.
The disk drive system performs three major operations: format, write and read. The disk drive can format the disk for storing data in the format supported by the system and the disk. The disk drive system also performs write operations to the disk to store data on the disk transmitted from the host computer to the disk drive system and read operations from the disk to read data from the disk and transmit it to the host computer. The disk is continuously rotating within the disk drive system, even as the read/write head moves between zones, which may cause the read/write head to cross over one or more servo fields as it travels from one track or zone to another track or zone. The controller circuit knows the angular position of the read/write head, but as the read/write head changes tracks between different zones, the number of sectors between servo marks changes, because of the change in bit density between zones. The controller does not know how to translate this angular position after a zone change into a known position on the track relative to the sectors and may be unable to determine the relative positioning of the sector pulses on the specific track. To re-orient itself on the disk, the hard disk controller will normally wait for the INDEX mark and orient itself from the INDEX mark for the track that it is on. However, waiting for the INDEX mark during a read or write operation increases the latency in reaching the next desired sector. This delay adds to the seek latency of the disk drive system.